Letter dated 8.1.1820 from Niels Rosenkrantz to Irgens Bergh, cf. Sass: Op. cit., II, p. 2 (“Thorvaldsen has now almost finished the busts of the princess and her sister. Those of the King and the Queen as well. The work is very successful.”)
Other references
Frederik Barfod: Thorvaldsensk Album, Copenhagen 1844, p. 48.
Else Kai Sass: Thorvaldsens Portrætbuster, Copenhagen 1963-65, Vol. I, p. 22, 470; Vol. II, p. 2-7, 10, 17, 180, 182.